Interconnects for use in integrated circuits are generally made of aluminum but aluminum is unsatisfactory in high speed semiconductor devices. The resistivity of aluminum as well as its electromigration limits and restrict its use in such applications. The present invention contemplates use of high purity copper as interconnects for use in integrated circuits which have requirements of line width about 0.18 .mu.m or less and are to be used in semiconductor devices at 1200 Mhz clock speeds. The interconnect lines are created by sputtering high purity copper from sputtering target assemblies.
To produce very small line widths, as described, we have determined that the sputtering target should be of high purity, i.e. at least 99.999 wt. % purity, (referred to as 5N copper) and preferably 99.9999 wt. % purity (referred to as 6N copper). Purity is important to maintain the low resistivity of the copper line so that high speed goals can be achieved. Moreover, purity may also influence electromigration resistance.
We have also determined that it is also desirable that the sputtering target have a fine and substantially uniform grain size, preferably about 50 .mu.m or less. The fine grain size aids in achieving a uniform film thickness during deposition, allows the target to sputter faster, and seems to result in fewer particle problems on the substrate, i.e. wafer. It is also important in accordance with the invention for the target to be diffusion bonded to a light weight, high strength backing plate. As targets become larger, particularly those designed for 300 mm diameter silicon wafers, the weight of the target becomes a significant handling factor. Technically, it would be possible to decrease the thickness of the sputtering target surface because only a part of the target is consumed. However, in order to take advantage of this capability, it is necessary to bond a thinner target blank onto a lightweight backing plate provided the thinness of the sputtering target surface does not allow the target assembly to warp during use. Warping of the target can lead to inconsistent deposition as well as particle generation. Therefore, it is important that the backing plate be strong and stiff in addition to being lightweight. The present invention achieves these objectives by providing a sputtering target assembly comprising high purity copper target diffusion bonded to a precipitation hardened aluminum backing plate. By use of diffusion bonding to join the target to the backing plate it is possible to avoid the need for soldering which is undesirable because temperatures required for sputtering often are sufficiently high to melt the solder bond of the targets and, moreover, the heat generated has potential to continue grain growth in the target after long periods of use.